Melting Glaciers Alter Earth's Gravity

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Melting glaciers can alter Earth's gravity field, scientists have
found, a discovery that is shedding light on when Greenland and
Antarctica began heavily melting.

Knowing the timing of this melting could help climate scientists
make better estimates of the potential sea level rise resulting from
melting ice pouring off these two massive ice sheets.

Anything that has mass has a gravity field that attracts objects
toward it. The strength of this field depends on a body's mass.
Since the Earth's mass is not spread out perfectly evenly, this
means its gravity field is stronger at some places and weaker in
others.

Round rebound

Scientists were investigating the Earth's geoid, or the average
gravity field across the globe. Over the past 20,000 or so years,
the geoid should have been become rounder just as the planet has
—the vast glaciers that once covered large swaths of the
continents withdrew at the end of the last Ice Age, and since then
the land that was once covered in ice has rebounded without
the weight of ice pressing down on it, giving the Earth a more
spherical shape.

However, starting about 20 years ago, scientists began noticing
this rebound was getting offset by some unknown factor, causing
the Earth's gravity field to stop changing its shape. To help
solve this mystery, researchers recently analyzed gravity mapping
data from the Gravity Recovery and Climate Experiment (GRACE)
satellite.

The investigators discovered that water from ice melting in
Greenland and Antarctica is apparently altering the Earth's gravity field. Mass once
concentrated as ice at the poles is flowing into the oceans
and spreading fairly quickly across the globe.

"Although these changes are very small, they can be detected by
satellites," said researcher R. Steven Nerem, a satellite
geodesist at the University of Colorado at Boulder.

This meltwater is drawn away from the poles toward the equator
because of the rotation of the Earth. (Imagine a ball of pizza
dough — it gets flatter when you spin it, as dough flies
outward.) The result is a bulge around Earth's middle, balancing
out any rounding effect from the withdrawal of glaciers.

Pinpointing melt's start

By figuring out how much the rounding effect has been offset,
scientists can potentially determine when the effect started, and
thus, when the melting began.

"Glaciologists know that Greenland and Antarctica are melting, but
it is harder for them to pin down when the melting started to
change — they just don't have much data prior to the
mid-1990s," Nerem told OurAmazingPlanet.

Understanding when changes in melting began is vital for
predicting how this effect might proceed in the future.

"Our study helps narrow the time uncertainty somewhat — it says
that the mid-1990s is roughly when the behavior of the ice sheets
started to change."

The scientists detailed their findings online July 13 in the
journal Geophysical Research Letters.